The 3GPP (3rd Generation Partnership Project) which is an international mobile communication standardization group has started the standardization of LTE-Advanced (Long Term Evolution-Advanced, LTE-A) as a fourth generation mobile communication system. As disclosed in Non Patent Literature 1, in LTE-A, a relay technology of relaying radio signals by using a relay node (RN) has been studied with the goals of coverage expansion and capacity improvement.
The relay technology will be described with reference to FIG. 12. FIG. 12 is a diagram illustrating a system which relays radio signals using the relay technology. In FIG. 12, eNB represents a base station, RN represents a relay node, and UE represents a radio communication terminal. Further, UE1 represents a radio communication terminal connected to eNB, and UE2 represents a radio communication terminal connected to RN.
Here, in LTE-A, RN having an individual cell ID as in eNB is being studied, and thus, when viewed from UE, RN can also be regarded as one cell like eNB.
eNB is connected to a network by wired communication, whereas RN is connected to eNB by wireless communication. A communication channel connecting between RN and eNB is called a backhaul channel. On the other hand, a communication channel connecting between eNB or RN and UE is called an access channel.
A radio relay system in a downlink channel (Down Link, DL) will be described with reference to FIG. 12. FIG. 12 is a diagram illustrating a radio relay system in the related art. RN receives signals from eNB in the backhaul channel. Further, RN transmits signals to UE2 in the access channel of RN.
Here, when the backhaul channel and the access channel are allocated in the same frequency bandwidth, if RN performs transmission and reception at the same time, loop-back interference occurs. For this reason, RN cannot perform transmission and reception at the same time. Thus, in LTE-A, a relay method is being studied in which the backhaul channel and the access channel of RN are allocated while being divided by the time domain (on a subframe basis).
A relay method in the related art in which a backhaul channel and an access channel of RN are allocated while being divided by the time domain (on a subframe basis) for allocation will be described with reference to FIG. 13. FIG. 13 is a diagram illustrating a subframe configuration of a downlink channel in the relay method in the related art. Reference signs [n, n+1, . . . ] in FIG. 13 represent subframe numbers. Boxes in FIG. 13 represent subframes of the downlink channel, and represent transmission subframes of eNB, reception subframes of UE1, transmission subframes of RN and reception subframes of UE2.
As shown in FIG. 13, eNB transmits signals in all the subframes. Further, UE1 receives signals in all the subframes. Further, as shown in FIG. 13, RN transmits signals in the subframes except for the subframe numbers [n+2, n+6]. UE2 can receive signals in the subframes except for the subframe numbers [n+2, n+6]. Furthermore, RN receives signals from eNB in the subframes of the subframe numbers [n+2, n+6].
As described above, in RN, the subframes of the subframe numbers [n+2, n+6] serve as the backhaul channel of RN, and the other subframes of the subframe numbers [n, n+1, n+3, n+4 and n+5] serve as the access channel of RN.
However, if RN transmits no signal in the subframes where RN serves as the backhaul channel, a problem occurs that a measurement operation of measuring the quality of RN does not function at UE of LTE which has not ascertained the presence of RN.
As a method of solving this problem, in LTE-A, using an MBSFN (Multicast/Broadcast over Single Frequency Network) subframe defined in LTE is being considered.
The MBSFN subframe is a subframe which is prepared to realize an MBMS (Multimedia Broadcast and Multicast Service) service in the future. The MBSFN subframe is designed to transmit cell-specific control information at the first two symbols and transmit signals for the MBMS in the domains of the third and subsequent symbols of the MBSFN subframe.
Here, the LTE terminal is capable of performing measurement by using the first two symbols in the MBSFN subframe. Thus, the MBSFN subframe is used in a pseudo-manner in the RN cell, and RN is capable of using the MBSFN subframe as the reception subframe of the backhaul channel. Specifically, RN transmits the control information specific to the RN cell at the first two symbols of the MBSFN subframe, and does not transmit data for the MBMS but receives signals from eNB in the domains of the third and subsequent symbols of the MBSFN subframe.
In this description, the MBSFN subframe as mentioned above will be called an “MBSFN subframe that RN uses as the backhaul”.
In this regard, in a mobile communication system, a situation occurs in which, when UE communicates with a certain eNB, received power from eNB is lowered due to movement of UE, change in the surrounding environment or the like and thus UE cannot maintain communication with eNB.
To cope with such a situation, UE can be re-connected to eNB or RN which is higher in received power than eNB in communication therewith, to thereby maintain communication. This is called handover.
Hereinafter, eNB or RN will be also called a “cell”, and a cell which communicates with UE will be also called an “own cell”.
In order to perform the handover, it is necessary that UE measures signal power from a cell which is present in the vicinity of the cell in communication therewith (the cell which is present in the vicinity of the own cell may be called a neighbor cell). In the 3GPP LTE, a process of measuring signal power or signal quality from this neighbor cell is called measurement.
In the measurement, a cell instructs UE to measure received power or quality from a neighbor cell, and UE measures the received power from the neighbor cell and notifies the own cell of the measurement result. UE performs the measurement using a reference signal (RS) or a synchronization signal generated on the basis of a cell-specific series.
In the measurement of LTE, as disclosed in Non Patent Literature 2, UE measures RSRP (Reference Signal Received Power) or RSRQ (Reference Signal Received Quality) using a cell-specific reference signal.